Resonant quadrifilar helical antennas were first developed by C. C. Kilgus in the late 1960's (see Kilgus, C. C., "Resonant Quadrifilar Helix," IEEE Transactions on Antennas and Propagation, Volume AP-17, May 1969, pages 349-351). In succeeding years this type of antenna has been used in many different applications including hand-held user terminals for satellite communication systems. It is particularly well suited to this application because its radiation pattern can be tailored to the required overhead coverage volume and because its pattern is relatively independent of the shape of any associated ground structure.
However, quadrifilar helix antennas may provide limited bandwidth in such applications. Although the bandwidth of the quadrifilar helix typically is as good as any other small resonant antenna occupying an equivalent volume, the natural bandwidth of the helix is often inadequate. Although the antenna's bandwidth may be increased by increasing the volume occupied by the antenna, this approach may be limited for mechanical and aesthetic reasons.
A very similar problem may occur when the communication system is designed to operate in two separate bands. For example, the antenna of a terminal may need to operate in transmit and receive bands which are separated by an unused band. The antenna may have adequate bandwidth for one band, but may not be adequate to encompass the other band. This represents a minor variation of the basic problem of inadequate bandwidth. It can be addressed by circuit concepts that solve the problem of inadequate bandwidth.
The above-described problem may be compounded by the need to provide a feed circuit. The feed circuit may consume some of the volume that could otherwise be used by the radiating structure. The volume of the feed circuit is often dominated by some form of balun structure. As indicated in Johnson's Antenna Engineering Handbook, most baluns tend to have at least one dimension equal to a quarter wavelength or more. The other two dimensions of a balun are often in excess of 0.05 wavelength, especially in a four-phase balun of the type often used in the feed of a circularly polarized antenna. A feed circuit including such a balun may consume 25 to 50% of the available volume. There are small transformer baluns commercially available that could potentially solve the problem, but many of these have been found to perform rather poorly in terms of insertion loss, amplitude balance, and phase balance. In addition, attempting to reduce the size of the balun often introduces an unwanted inductive susceptance across the feed points of the antenna.
A conventional approach to providing broad band or dual band capability is described in Johnson's Antenna Engineering Handbook. This approach includes the addition of a parallel resonant circuit to a series resonant antenna, such as the quadrifilar helix, to increase the bandwidth of the antenna. The same circuit can provide a dual band response if the resonant resistance of the antenna and the capacitance and the inductance of the parallel resonant circuit are appropriately adjusted. Although the addition of a parallel resonant circuit can be quite effective, the capacitance needed tends to be relatively large and the inductor relatively small, which may make it difficult to utilize standard components.
Another solution to the balun size problem is to simply eliminate the balun. The resulting antenna circuit may still function reasonably well, but the unbalanced feed typically causes currents to flow on the outer conductor of the coaxial feed, thereby skewing the radiation pattern and causing the pattern to vary as a function of frequency. These effects may be unacceptable in some applications.